This paper overviews the development of digital beam-forming passive millimetre wave (PMMW) imagers at the frequencies of 22.51 GHz, 91.65 GHz, and 183.31 GHz. These systems are demonstrators for imager architectures that can evolve into security screening products, all weather flying aids, satellite based imagers for planetary observation and test beds for future radio astronomy hardware. These are all areas where the common goal is to develop an imaging capability which is highly sensitive, has good angular resolution and has a minimal volume, typically a factor of 100 smaller than equivalent quasi-optical systems, and minimal weight. This paper describes the science behind electronic beam-forming for passive imaging, the objectives of the project, its risks, and the future of the technology.